It has been found that the use of distributed power supplies, i.e., placing a plurality of power converters close to the loads in an electronic system instead of using one centralized power supply, improves the performance of the electronic system. There are several reasons for this improved performance. One of the reasons is that the transient response to a sudden change in load degrades as the distance between the power converter and the load increases. The degradation is introduced by the resistive and inductive effects inherent in the conducting cable connecting the power converter and the load. If the power converter is placed close to the load, the length of the cable decreases thereby improving the transient response. Another reason is that each power converter in the distributed power supply system could be designed to match the requirements of its corresponding load while the design of a centralized power supply necessarily introduces compromises.
One of the requirements for placing a power converter close to a load is that the power converter must have a dimension smaller than the available space surrounding the load. Many modern electronic systems place cards populated with electronic elements in slots close to each other. Thus, the power converter should have a low profile because its height preferably should be smaller than the distance between the cards.
The power transformer is one of the largest components in a power converter. Many components used in a power converter have physically shrunk due to the improvements in materials, availability of specialized integrated circuits, surface mount packaging that enables the surface mounting of components on printed circuit boards, and improvements in circuit design. Likewise, the physical size of a power transformer has shrunk due to the increased switching frequency, typically around 1 MHz, and the availability of more efficient ferrite core materials. However, it is still desirable to reduce the physical size of a power transformer further.
There are problems associated with switching a power transformer at a high frequency and reducing the size of the power transformer. A higher switching frequency increases conduction loss in the transformer's windings because the conduction loss due to skin effect and proximity effect increases with frequency. A higher rate of change in operation flux also increases both the hysteresis loss as well as eddy current loss in the core. These losses are transformed into thermal energy. The ability to dissipate thermal energy is proportional to the surface area of the power transformer. As the physical dimension of the transformer is reduced, the ability to dissipate thermal energy decreases, thereby increasing the risk that the temperature of the power transformer will rise above the transformer's maximum allowable operating temperature.
Another problem with reducing the size of a power transformer is that there may not be sufficient space in the transformer for accommodating insulating material. As a result, the isolation between the primary and the secondary windings is reduced. The safety requirements for a transformer connected to an AC line are governed by UL 1950 and IEC 950. Both regulations required that the creepage distance, i.e., the shortest distance between two conducting parts of the primary and the secondary winding measured along the surface of the insulating material between them, be at least 5 mm. In addition, the insulation between the primary and the secondary windings must have a minimum thickness of 0.4 mm and be able to withstand a Hi-Pot test of 3000 VAC. As the size of a power transformer is reduced, it becomes more difficult to satisfy these safety requirements.
A further problem associated with reducing the size of a power transformer is that the electromagnetic coupling between the primary and the secondary windings may be reduced. The electromagnetic coupling between these two windings is related to the amount of magnetic flux generated by the primary winding which reaches the secondary winding. The size and shape of the primary and the secondary windings may not be optimal for electromagnetic coupling due to the reduction in size of the power transformer.